1. Field of the Invention
The present invention relates to an image forming method for fixing a toner image in place on a recording medium with heat and pressure, which is applicable to electrophotographic image forming apparatuses such as copiers, printers, and facsimile machines.
2. Description of the Background
When toner particles adhering to the entire surface of a relatively thin sheet of paper, that is, from the leading edge to the trailing edge, are fixed thereon in a fixing device of an electrophotographic printing apparatus, disadvantageously, such thin paper is likely to get jammed in the fixing device or wrap around a fixing member.
To prevent such paper jam or wraparound and facilitate separation of the paper from the fixing member, one proposed approach involves reducing the diameter of the fixing member to increase the curvature thereof. Another approach involves applying oil to the fixing member to provide a release layer between the fixing member and toner particles. Further, another approach involves including a release agent (e.g., a wax) in toner particles.
As yet another approach, Japanese Patent Application Publication Nos. 2008-20821, 2005-284089, and 2001-265146 each disclose a fixing device including a fuser roller and a pressure roller each having two or more convex and concave portions in the axial direction. A fixing nip is defined by engaging the convex portions of the fuser roller and the concave portions of the pressure roller, and engaging the concave portions of the fuser roller and the convex portions of the pressure roller.
However, such a fixing device does not solve the problem of paper jam or paper wraparound when a recording medium is a relatively thin sheet of paper or toner particles are adhering to the entire surface of the recording medium from the leading edge to the trailing edge thereof.
On the other hand, the above fixing devices do not have any problem in terms of image gloss. Although the gloss of the first resulting image and that of succeeding resulting images may be slightly different due to a decline in the fuser roller temperature, the gloss is uniform throughout the entire resulting image because the entire surfaces of both the fuser and pressure rollers have a constant temperature.
In attempting to facilitate separation of a recording sheet, especially a relatively thin sheet of paper, from a fixing member, there is also an approach different from the above-described examples in which an effort is made to improve flexural stiffness of the recording sheet. For example, a related art fixing device includes a fuser roller having convex and concave potions that defines an undulating surface and a pressure roller having convex and concave potions that defines an undulating surface, to improve flexural stiffness of a recording sheet that passed through the fixing nip defined between the fuser roller and the pressure roller.
However, such a fixing device is likely to cause gloss difference in a stripe pattern with respect to the direction of feed of the recording sheet. In particular, an image portion which passed through the convex portion of the fuser roller is likely to have extremely low gloss.
One possible reason for this is considered as follows. Generally, the following inequations are satisfied:Rt>RbRtω>Rbωwherein Rt represents a distance between the center of the fuser roller and the top of the convex potion in a longitudinal cross-section, Rb represents a distance between the center of the fuser roller and the valley of the concave potion in a longitudinal cross-section, ω represents an angular speed of the fuser roller, Rtω represents a linear speed of the top of the convex portion of the fuser roller, and Rbω represents a linear speed of the valley of the concave portion of the fuser roller. It means that the linear speed of the top of the convex portion is greater than that of the valley of the concave portion.
When a recording sheet passes through a fixing nip defined between such fuser and pressure rollers with a surface having different linear speeds by location, the recording sheet meets frictional resistance from the rollers. Therefore, a toner image is fixed on the recording sheet at a slightly lower speed than the average linear speed of the surface of the fuser and pressure rollers. A toner image portion which passes through the convex portion of the fuser roller receives a shear force in a sheet movement direction because the convex portion has a greater linear speed than the recording sheet. In a case where the aggregation force of the toner balances the shear force, the toner image is likely to attract to the fuser roller when being fixed on the recording medium, thereby decreasing image gloss.
What is needed, then, is a method of simultaneously providing both trouble-free separation of paper from roller and superior image gloss.